Since nitride semiconductors have characteristics such as exhibiting high saturated electron velocity and a wide band gap, application of nitride semiconductors to semiconductor devices having high withstand voltage and high power has been studied on the basis of utilization of such characteristics. For example, the band gap of GaN that is a nitride semiconductor is 3.4 eV and larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV); thus, GaN exhibits high breakdown field strength. GaN is therefore a highly practical material used for power semiconductor devices which operate at high voltage and which output high power.
Semiconductor devices utilizing nitride semiconductors, such as field effect transistors, have been reported, in particular, high electron mobility transistors (HEMTs). Among HEMTs utilizing GaN (GaN-HEMTs), for instance, an AlGaN/GaN-HEMT in which GaN is used for an electron transit layer and in which AlGaN is used for an electron supply layer is attracting attention. The AlGaN/GaN-HEMT is expected to be applied to highly efficient switching devices and power devices used for electric vehicles.
Related art is disclosed in Japanese Laid-open Patent Publication Nos. 2009-170746 and 2008-4720.
In nitride semiconductor devices, a technique for controlling local generation of two dimensional electron gas (2DEG) is demanded. In view of so-called fail safe, for example, HEMTs desirably operate in a normally-off mode in which electric current does not flow in the case where gate voltage is not applied.
In existing HEMTs such as AlGaN/GaN-HEMTs, surfaces (upper surfaces) of GaN and AlGaN used for an electron transit layer and an electron supply layer, respectively, are c-planes (0001) or are m-planes (1-100) or a-planes (11-20).
In an AlGaN/GaN-HEMT having the former structure, the gate electrode, the source electrode, and the drain electrode are formed on the c-plane that is a polar face. A difference in a lattice constant between GaN and AlGaN generates distortion in AlGaN, which causes piezoelectric polarization and spontaneous polarization of AlGaN. Since a channel in a transistor is formed along such a polar face, high-concentration 2DEG is generated owing to the piezoelectric polarization and the spontaneous polarization. In this case, however, the high-concentration 2DEG in the channel causes the flow of gate current even though the gate voltage is not applied, and a negative voltage is therefore applied to a gate electrode to interrupt the gate current. This phenomenon is operation in a normally-on mode; thus, there is a problem in that it is difficult to enable desired operation in a normally-off mode.
In an AlGaN/GaN-HEMT having the latter structure, the gate electrode, the source electrode, and the drain electrode are formed on an m-plane or an a-plane that is each a non-polar face. Since a channel is formed along such a non-polar surface, piezoelectric polarization and spontaneous polarization are not caused. 2DEG is not generated in the channel in the case where the gate voltage is not applied, and gate current does not flow; hence, operation in a normally-off mode is enabled. In this case, however, the absence of 2DEG in the channel increases on-resistance, which becomes problematic.
Furthermore, there is a problem common to AlGaN/GaN-HEMTs having the former or latter structure. In order to provide high withstand voltage that is a requirement for power devices, the length Lgd between the gate electrode and the drain electrode is increased.
An increase in the length Lgd unfortunately leads to an increase in the size of a device, which restricts the number of devices that may be integrated. Although the demand for a power device, such as an AlGaN/GaN-HEMT, having a fine structure and enabling high integration has been increased in recent years, it has been difficult for existing AlGaN/GaN-HEMTs that have channels formed along a polar or non-polar face to satisfy such demands.